To evaluate genetic influences on the use and abuse of alcohol, we compared questionnaire measures of the frequency, quantity, and density of social drinking, and the frequency of alcohol-induced passouts self-reported by 879 monozygotic (MZ) and 1940 dizygotic (DZ) pairs of twin brothers, aged 24-49 yr. The measures of frequency, quantity, and density (heavy drinking once or more a month) significantly intercorrelate, and the self-reported alcohol consumption by this sample is satisfactorily stable and consistent with nationwide sales figures. None of the drinking measures was associated with twin type (zygosity), and only density correlated with age. Similarity of drinking habits among twin brothers was evaluated as a function of their genetic resemblance and age, the frequency of their social contact with one another, and the interactions of these terms. The effects were estimated from hierarchical linear regressions of a double-entry data matrix from which each twin's drinking was predicted from that of his twin brother, and that pair's age, zygosity, cohabitation status, and frequency of social contact. Significant genetic variance was found for each of the drinking measures with heritability estimates ranging from 0.36 to 0.40. Co-twins in more frequent social contact with one another reported greater similarity in their use of alcohol, but heritable variance remained after the effects of age and social contact were removed from both mean levels and co-twin resemblance. Reported frequency of passouts yielded significant, but equivalent, correlations in both MZ and DZ twins and no evidence of genetic influence.